microbialite formation in seawater of increased alkalinity, satonda crater lake, indonesia

23 Pages·2002·7.88 MB·English

MICROBIALITE FORMATION IN SEAWATER OF INCREASED ALKALINITY,
SATONDA CRATER LAKE, INDONESIA
GERNOTARP,ANDREASREIMER,ANDJOACHIMREITNER
GeowissenschaftlichesZentrumderUniversita¨tGo¨ttingen,AbteilungGeobiologie,Goldschmidtstraße3,D-37077Go¨ttingen,Germany
e-mail:[email protected]
ABSTRACT: The crater lake of the small volcanic island Satonda, In- 1986) but also in its entire major ion composition (Spencer and Hardie
donesia,isuniqueforitsred-algalmicrobialreefsthrivinginmarine- 1990;Hardie1996;Arpetal.2001;Lowensteinetal.2001).
derivedwaterofincreasedalkalinity.Thelakeisapotentialanalogue Nonetheless, the significance of absolute ion concentrations,alkalinity,
for ancient oceans sustaining microbialites under open-marinecondi- and CaCO supersaturation in microbialite formation remains a matter of
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tions.Currentreefsurfacesaredominatedbylivingredalgaecovered discussion(e.g.,KempeandKazmierczak1990a,1990b,1994;Grotzinger
bynon-calcifiedbiofilmswithscatteredcyanobacteriaanddiatoms.Mi- 1990; Knoll et al. 1993; Grotzinger and Knoll 1995). This is especially
nor CaCO precipitates are restricted to the seasonally flooded reef trueforCa2(cid:49)andalkalinitybecausebotharestronglyaffectedbybiological
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tops, which develop biofilms up to 500 (cid:109)m thick dominated by the processes,suchasactiveCa2(cid:49)removalbycellularionpumpsandHCO (cid:50)
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cyanobacteriaPleurocapsa,Calothrix,Phormidium,andHyella.Micro- releasefrombacterialsulfatereduction.Weatheringandplatetectonicpro-
crystalline aragonite patches form within the biofilm mucilage, and cesses are two additional factorsaffecting seawatercomposition.Further-
fibrousaragonitecementsgrowinexopolymer-poorspacessuchasthe more,changesofpCO intheatmosphere(seeRoyeretal.2001forreview)
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inside of dead, lysed green algal cells,andreef frameworkvoids.Ce- havetobetakenintoconsiderationwhendiscussingconstraintsonancient
mentation of lysed hadromerid sponge resting bodies results in the seawater alkalinities and CaCO mineral supersaturation (e.g.,Mackenzie
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formationof‘‘Wetheredella-like’’structures. andPigott1981;KempeandDegens1985;MackenzieandAgegian1989;
Hydrochemistry data andmodel calculationsindicatethatCO de- Grotzinger 1990, 1994; Morse and Mackenzie 1998). Thus, in situ calci-
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gassing after seasonal mixis can shift the carbonate equilibrium to fying biofilms and microbial mats forming in modified seawater are of
cause CaCO precipitation. Increased concentrations of dissolved in- specialinterestaspotentialanaloguesforfossilmicrobialitesofopenma-
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organiccarbonlimittheabilityofautotrophicbiofilmmicroorganisms rine settings. Knowledge of their formation processes may provide indi-
to shift the carbonate equilibrium. Therefore,photosynthesis-induced cationsforthereconstructionoftheambientseawaterchemistry.
cyanobacterialcalcificationdoesnotoccur.Instead,passive,diffusion- Aquasi-marinealkalinelakesustainingmicrobialiteformationhasbeen
controlledEPS-mediatedpermineralizationofbiofilmmucusatcontact described from Satonda (Kempe and Kazmierczak 1990a, 1990b, 1993;
with the considerably supersaturated open lake watertakesplace.In Kempeetal.1996,1997),asmallvolcanicislandnorthofSumbawa,In-
contrasttoextremesodalakes,thereleaseofCa2(cid:49)fromaerobicdeg- donesia (Fig. 1). These microbialites form a part of the red-algal-micro-
radationofextracellularpolymericsubstancesdoesnotsupportCaCO bialite reefs, which occur at protrusions of the rocky lake shore (Fig. 1).
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precipitation in Satonda because the simultaneously released CO is Kazmierczak and Kempe (1990) suggested that the microbialites formed
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insufficientlybuffered. bycalcifyingcyanobacterialmatsandcomparedthemwithPaleozoicstro-
Subfossil reef parts comprise green algal tufts encrustedbymicro- matoporoids. They further argued that this similarity was a reason to be-
stromatolites with layers of fibrousaragoniteand anamorphous,un- lievethatearlyPaleozoicseawaterhadahigheralkalinityandCaCO su-
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identifiedMg–Siphase.Themicrostromatolitesprobablyformedwhen persaturationthanmodernseawater.
LakeSatondaevolvedfromseawatertoCa2(cid:49)-depletedraised-alkalinity The purpose of this paper is to elucidate mechanisms of microbialite
conditions because of sulfate reduction in bottom sedimentsandpro- formationinseawaterofincreasedalkalinity.Wefocusonprecipitationin
nounced seasonality with deep mixing events and strong CO degas- recent biofilms in the lake in relation to biofilm structure and seasonal
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sing.Thelattereffectcausedrapidgrowthoffibrousaragonite,while hydrologiccycleinSatonda.Investigationofmechanismsofrecentbiofilm
Mg–Si layers replaced the initially Mg-calcite-impregnated biofilms. calcification serves as a basis for exploring the formation of the unique
Thiscouldbeexplainedbydissolutionofsiliceousdiatomsandsponge subfossil microbialite. Finally, results of the present study are compared
spicules at high pH, followed by Mg-calcite dissolution and Mg-silica andcontrastedwiththemodelofSatondaLakemicrobialiteformationpro-
precipitation at low pH due to heterotrophic activity within the en- posed by Kazmierczak and Kempe (1990, 1992) and Kempe and Kaz-
tombedbiofilms. mierczak(1990a,1990b,1993).
FACTORS CONTROLLINGBIOFILM CALCIFICATION
INTRODUCTION
Biofilms consist of microbial cells, mainly prokaryotes of severalmet-
Normal-marinesettingstodaysustainmicrobialiteformationonlyinex- abolicgroups(VanGemerden1993),embeddedinahighlyhydratedmu-
ceptional cases.Thereisonly oneknown exampleof lithifyingstromato- cilagecomposedofextracellularpolymericsubstances(EPS)(Decho1990;
lites (Bahamas) in an open marine setting of normal seawater salinity Wingenderetal.1999).Incontrasttobiomineralizationineukaryoticalgae
(Dravis 1983; Dill et al. 1986; Reid et al. 2000; Visscher et al. 2000). andmetazoa(Westbroeketal.1984;AddadiandWeiner1985;Mann1988;
However, many marine fossil microbialites differ from the agglutinated LowenstamandWeiner1989;SimkissandWilbur1989),precipitationin
Bahamianstromatolitesinthattheycontainlesstrappedparticlesandsub- biofilmsisrarelycontrolledbythemicroorganismsandisregardedasin-
stantially more in situ precipitated carbonate (Gebelein 1976; Grotzinger ducedormediated(e.g.,Pentecost1991;Riding1991a,2000).Indeed,non-
1990;Riding1991a,2000)—similartothestromatolitesofthenonmarine living organic matter can mineralize without apparent direct involvement
typelocality(Kalkowsky1908;PaulandPeryt1999).Onereasonforthis of living cells, a process known as ‘‘organomineralization’’ (Trichet and
discrepancy between recentandpre-Tertiarymarinecarbonatesedimenta- De´farge1995;De´fargeetal.1996;seealsoReitneretal.1995).
tionisprobablyachangeinoceanchemistry.Seawaterchemistrychanged Therearethreemajorfactorsthataresignificantinbiofilmcalcification.
through time not only with regard to the Mg/Ca ratio (Wilkinson 1979; First,istheinitialdissolvedinorganiccarbonpoolandsaturationstatewith
Riding1982;Sandberg1983;Wilkinsonetal.1985;WilkinsonandGiven respecttoCaCO minerals.Approximatelyten-foldcalcitesupersaturation
3
JOURNALOFSEDIMENTARYRESEARCH,VOL.73,NO.1,JANUARY,2003,P.105–127
Copyright(cid:113)2003,SEPM(SocietyforSedimentaryGeology) 1527-1404/03/073-105/$03.00
106 G.ARPETAL.
FIG. 1.—A) Location of the Satonda island, north of Sumbawa, Sunda archipelago, Indonesia. B) Bathymetric map of the Satonda Crater Lake (from Kempe and
Kazmierczak1990,modified).Arrowsindicatereefsitessampledinthisstudy.C)Schematicsectionofthered-algal-microbialitereefs.Drawingisnottoscale.
(i.e.,SI (cid:53)1.0;SI (cid:53)0.86;fordefinitionofSIseeTable1)seemsto low-DICsettings,whereashigh-DICsettingsarealmostunaffected(Arpet
Cc Arag
be a prerequisite for biofilm calcification (Arp et al. 1999a; Arp et al. al.2001).
1999b, 2001). This level of threshold supersaturation for CaCO precipi- The second major factor in biofilm calcification are physiological pro-
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tationvariesindifferentsettings,mainlydependentontheMg2(cid:49),SO 2(cid:50), cesses of microorganisms; theseprocessescould alterthe carbonateequi-
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andPO 3(cid:50)concentrations.Startingfromthelevelofinitialsupersaturation, librium and Ca2(cid:49) concentration in the microenvironment. Physiological
4
the effect of carbon fixation by organisms on the CaCO supersaturation processes capable of inducing CaCO precipitation (i.e.,temporarilyshift
3 3
depends on the concentration of dissolved inorganic carbon (DIC). The the SI to values higher than 1.0) are autotrophic CO fixation, nitrate
Cc 2
same amount of fixed carbon causes a great change in supersaturationin reduction and ammonification, sulfate reduction, and coupled sulfate re-
TABLE1.—WaterchemistryofsamplesfromthreedifferentwaterdepthsthatarerepresentativeofthethreelakewaterbodiesofSatondaCraterLake,respectively.
TotalAlkb DICc Ca2(cid:49) Mg2(cid:49) pCOf
Sample Depth T(cid:56)C pH p(cid:171)a Salinity‰ meqL(cid:50)1 mmolL(cid:50)1 mmolL(cid:50)1 mmolL(cid:50)1 SI d SI d (cid:109)atm2
Cc Arag
SamplingperiodOctober1993
Mixolimnion(0–22m) 0.1m 30.7 8.58 4.52 31.4 4.17 3.41 4.64 42.50 1.00 0.86 282
5m 30.9 8.59 4.38 31.4 4.15 3.38 4.58 42.58 1.00 0.86 269
Monimolimnion,concentratedlayer(22–50m) 30m 29.8 7.28 (cid:50)1.92 37.3 7.38 7.74 5.53 49.88 0.19 0.05 14791
Monimolimnion,brine(50–69m) 60m 29.4 6.94 (cid:50)2.94 41.7 50.43 56.45 5.93 57.58 0.68 0.54 218776
SamplingperiodJune1996
Mixolimnion(0–24m) 0.5m 30.6 8.50 6.31 29.4 3.97 3.33 4.55 43.57 0.92 0.78 339
5m 30.5 8.58 6.31 29.4 4.04 3.30 4.65 43.28 0.99 0.85 269
Monimolimnion,concentratedlayer(24–51m) 30m 29.7 7.35 (cid:50)2.12 37.2 7.60 7.89 5.93 51.39 0.29 0.15 12589
Monimolimnion,brine(51–70m) 60m 28.9 6.97 (cid:50)3.54 41.6 47.56 52.87 6.30 58.25 0.71 0.57 186209
Standardseawaterb 25.0 8.22 8.45 35.0 2.406 2.18 10.66 55.07 0.76 0.61 417
aRedoxintensityp(cid:171)(cid:53)(cid:50)log{e(cid:50)}
bTotalalkalinity(cid:53)acid-neutralizingcapacityexpressedasmilliequivalentperliter
cDissolvedinorganiccarbonDIC(cid:53)[CO(aq)](cid:49)[HCO](cid:49)[HCO(cid:50)](cid:49)[CO2(cid:50)]
dSI ,SI :Saturationindexforcalcite2andaragonit2e;S3I(cid:53)log(IA3P/K),wher3eIAP(cid:53)ionactivityproductofCa2(cid:49)andCO2(cid:54)s,andK(cid:53)solubilityproductofcalciteandaragonite,respectively.
Cc Arag 3
efromNordtrometal.(1979).
fPartialpressureofcarbondioxide.
MICROBIALITEFORMATIONINSATONDACRATERLAKE 107
duction–methanotrophy (e.g., Berner 1971; Golubic´ 1973; Kelts and Hsu¨ waters is raised significantly, thereby lowering pHand CaCO supersatu-
3
1978;Krumbein1979;Lyonsetal.1984;ThompsonandFerris1990;Rit- ration to levels (SI (cid:53) (cid:49)0.19 to (cid:49)0.29, SI (cid:53) (cid:49)0.05 to (cid:49)0.15)
Cc Arag
geretal.1987;Paulletal.1992;Fortinetal.1997;Castanieretal.2000; unfavorable for CaCO precipitation (Table 1). The anoxic brines of the
3
Peckmannetal.2001).Inbiofilms,anincreaseinsupersaturationisfacil- lowermonimolimnionalsoshowaraisedsalinity,andalsoatremendously
itatedbythereduceddiffusionrateswithinthemucilage. high pCO and alkalinity, so that CaCO supersaturation is raised to an
2 3
Finally, the third crucial step in biofilm calcification is the process of SI (cid:53) (cid:49)0.68 to (cid:49)0.71 and SI (cid:53) (cid:49)0.54 to (cid:49)0.57. None of the
Cc Arag
formation of seed crystals, which is controlled by the concentration and parameters of the carbonate system in the monimolimnion varied signifi-
stereochemicalarrangementofacidicgroupsinEPS(TrichetandDe´farge cantlybetweenOctober1993andJune1996(Table1).
1995;Arpetal.1998;Arpetal.1999a;Arpetal.1999b,2001;Kawaguchi
andDecho2001).Disorderedcomplexationofdivalentcations,character-
MATERIAL AND METHODS
isticofmanycarbohydratepolymers,shouldinhibitprecipitation.Bycon-
trast,organicmatricesinbiomineralizingorganismsshowwelldefinedcar- SamplesinvestigatedinthisstudyweretakenbySCUBAdivingduring
boxylategroupscorrespondingtothecrystallatticewhenattachedtosolid thedryseasoninOctober1993(Appendix1;seeAcknowledgments)and
substrates and therefore promote nucleation (Addadi and Weiner 1985; shortlyafterthewetseasoninJune1996(Appendix2).103hardpartthin
Mann 1988; Lowenstam and Weiner 1989; Simkiss and Wilbur 1989). sectionsof30biofilmsampleswerepreparedaccordingtomethodsinArp
However,inhibitioninmanypolyanionicorganicacidsisonly temporary et al. (1998) and Arp et al. (1999a). In addition, 30 conventional thin
(e.g.,Sikesetal.1994)andre-arrangementoftheacidicpolymersbyro- sectionsofdriedreef-rocksampleswerepreparedforpetrographicdescrip-
tation of exocyclic groups and around the glycosidic linkages (Brant and tion.EpifluorescenceimageswereobtainedbyusingaZeissAxioplanmi-
Christ1990)isassumedtoresultaccidentiallyinsuitablenucleationsites croscope equipped with a Peltier-cooled VISICAM-color CCD camera
inbiofilmEPSaftersaturationwithdivalentcations(Arpetal.1999a;Arp (PCOComputerOpticsGmbH,Kehlheim)(Manzetal.2000).Imagestacks
etal.1999b). withaZspacingof0.5or0.25(cid:109)mwereobtainedbyusingapiezo-mover
(PhysikInstrumenteGmbH&Co,Waldbronn)attachedtoa‘‘Plan-Apoch-
romat’’ 63(cid:51) objective (Zeiss, NA (cid:53) 1.4). Image processing and three-
ENVIRONMENTALSETTING
dimensionalrestorationwerecarriedoutbyusingtheMetamorph(cid:116)Imaging
Satonda,avolcanicisland2km(cid:51)3kminsize,issituated3kmnorth software(UniversalImagingCorporation,WestChester,Pennsylvania)and
ofSumbawa,Indonesia(Fig.1).Itbelongstotheinnerpartofthe6,000- theEPR(cid:121)deconvolutionsoftware(Scanalytics,Billerica,Massachusetts).
km-longSundaIslandArc,whichislinkedtothesubductionzonebetween Conventionallightmicroscopywascarriedoutusingthesamemicroscope.
Sumatra and the eastern Banda Sea. The island shows a central double Thechemicalcomposition(Ca,Mg,Sr,Si)ofthreesamples(including
calderathatformedafterthelasteruptionmorethan4000yrB.P.(Kempe subfossil carbonates, red algal–foraminiferal crusts, recent precipitates of
andKazmierczak1990a,1990b,1993;Kempeetal.1996,1997).Initially reef surfaces) was determined by electron microprobe analysis. Carbon-
filled with freshwater, the crater lake was flooded with seawaterapproxi- coated polished thin sections of LR-White-embedded samples wereused.
mately3000yrB.P.Today,thereisnoconnectiontothesurroundingsea. Theanalyseswereperformed at 15 kVand 12 nAon aJEOL JXA8900
Thelakelevelremains1–2mhigherthanthatofthesea,evenduringthe RLelectronmicroprobeattheInstituteofGeochemistry,Go¨ttingen.Fifty-
dry season. High organic input, intense sulfate reduction, and periods of fourspotmeasurementsandfivelinescans(166spotmeasurements)were
high evaporation changed the marine lake during the last few thousand performed to differentiate mineral phases. Ca, Mg, and Si were analyzed
years into an alkaline meromictic lake (Kempe and Kazmierczak 1990a, for16seconds,whereasSrwasanalyzedfor30seconds.Notethatdatain
1990b,1993;Kempeetal.1996,1997). wt%(oxides)refertowhole-rockcomposition,whereasmole%(CaCO ,
3
Water-chemistry data are available for the dry season of October1993 MgCO ) and ppm (Sr) refer to the carbonate phase. The detection limit
3
and the end of the wet season of June 1996 (Table 1). The lake level (limitofquantification)isgivenbyI (cid:53)t (P;f)(cid:51)(cid:115) ,wheret (cid:53)level
dl z BG z
fluctuatesinarangeof1mbetweentheseasons.Inprinciple,thelakeis ofsignificance,P(cid:53)confidencelevel(95%),f(cid:53)degreesoffreedom,and
divided by two chemoclines into an oxygenated mixolimnion, an anoxic (cid:115) (cid:53) standard deviation of the background intensity. Typical detection
BG
uppermonimolimnion(‘‘concentratedlayer’’)andananoxiclowermoni- limits are 0.06 wt % for CaO, 0.05 wt % for MgO, 0.08 wt % for SrO,
molimnion(‘‘brine’’).Furtherdetailedwater-chemistrydataarepublished and 0.42 wt % forSiO . The statisticalerrorwascalculatedby(cid:68)n%(cid:53)
2
inKempeandKazmierczak(1993)andKempeetal.(1996,1997).Bicar- ((cid:207)n/n) (cid:51) 100, where n denotes the absolute counts. Typical statistical
bonateproductionofthemonimolimnionsulfatereductionispartlytrans- errors are 0.17 wt % for CaO, 0.03 wt % for MgO, 0.02 wt % for SrO,
ferredtothemixolimnion,raisingalkalinityto4.04–4.15meqL(cid:50)1andpH and 0.01 wt % for SiO . The locations of measurement points werecon-
2
to8.6(KempeandKazmierczak1993).Asaconsequence,supersaturation trolled by epifluorescence microscopy. The craters in the samples caused
ofsurfacewaterswithrespecttocalciumcarbonatemineralsishigh(Table bytheelectronbeamwere10–15(cid:109)minsize.
1;SI (cid:53)(cid:49)0.92to(cid:49)1.00,SI (cid:53)(cid:49)0.78to(cid:49)0.86)comparedtostan- Hydrochemical calculations of saturation indices and modeling simula-
Cc Arag
dard seawater (SI (cid:53) (cid:49)0.76, SI (cid:53) (cid:49)0.61; Nordstrom et al. 1979). tions of seasonal lake cycle and EPS degradation were carried out using
Cc Arag
The salinity of the mixolimnion is 31.4 ‰ in the dry season, and drops thecomputerprogramPHREEQC(Parkhurst1995).Formass-balancecal-
slightlyto29.4‰attheendoftherainyseason(Table1).Seasonalrain culations,thevolumesoflakewaterlayersweredeterminedbyareamea-
precipitationlowerspH,alkalinity,andCa2(cid:49)ofthesurfacewatersatless surements (Metamorph(cid:116) Imaging software) of the Satonda crater lake
than1mdepth,buttheresultingCaCO supersaturationisstillhigh(SI bathymetric map published in KempeandKazmierczak(1993).Reefsur-
3 Cc
(cid:53)(cid:49)0.92,SI (cid:53) (cid:49)0.78)andonly slightlylowercomparedtothedry facebiofilmareahasbeendeterminedbyaddingaverticalcylindricalplane
Arag
season.TheMg2(cid:49)/Ca2(cid:49)molarratioofmixolimnionwatersvariesaround (corresponding to 0.3–0.9 m depth) to the horizontal area of flooded reef
10,thusfavoringcalciumcarbonatetoprecipitateasaragonite.Withregard tops, followed by multiplication by a roughness factor. The latter factor
topCO ,themixolimnionwatersareslightlyundersaturatedasaresultof has been calculated from surface morphology of reef-top thin sections.
2
algal photosynthesis, above all by the extensive green algal carpet. Only Biofilmvolumeofreeftopsectionwasdeterminedfrombiofilmthickness
the surface water pCO (0.5 m depth) of the rainy season is almost in (140 measurements in 7 thin sections) and reef surface area as described
2
equilibriumwiththeatmosphere(Table1). above. Thecalcifiedproportionofbiofilmswasascertainedin7thinsec-
The‘‘concentratedlayer’’ofthemonimolimnionshowsaraisedsalinity, tions from total biofilm area and calcified area in thin sections using the
whichisconstantbetweentheseasons(Table1).ThepCO oftheanoxic Metamorph(cid:116)Imagingsoftware.
2
108 G.ARPETAL.
RESULTS Electron microprobe analyses (Fig. 4) revealed that fibrous crust parts
consist of aragonite with 98.4 mole % CaCO and 8400 ppm Sr (i.e.,55
3
FaciesSuccessionofSubfossilReefCarbonates wt%CaO,1wt%SrO),whereascryptocrystallinetoamorphouspartsare
composedofanundeterminedphasewithan(cid:59)1:1molarratioofMgand
Nocomplete,continuoussectionthroughthered-algal-microbialitereefs
Si (36–45 wt % SiO , 15–29 wt % MgO). No high-Mg calcite has been
isavailable.Maximumtotalthicknessfromthevolcanicsubstraterockto 2
detectedwithinthemicrostromatoliticcrusts.
thelivingsurfaceisestimatedtobeapproximately1m(KempeandKaz-
Numerous organic remains are enclosed within the microstromatolitic
mierczak1993).Asuccessionofthreemajorfaciestypesisreconstructed crusts. Most striking are abundant, straight to curved filaments 1 (cid:109)m in
on the basis of blocks broken from the reefs. The contact with basement diameter and up to more than 200 (cid:109)m in length, which appear dark in
boulderswasobservedonlyatthesubaeriallyexposedreeftopsduringthe
transmitted light (Fig. 2B–D). They cross-cut the fibrous aragonite fabric
dryseasoninOctober1993.
andshowoccasional,irregularbranching.Farlessabundantarebrownish,
Inprinciple,thebaseofsuccessioniscomposedofaserpulidtubeframe- organic-walledspheres5(cid:109)mindiameterthatoccurisolatedoringroups
work(‘‘serpulite’’).Themajorpartofthereefisformedbyamicrobialite
of three to more than ten (Fig. 2E). Although enclosed in the aragonite,
encasing green-algal molds. This facies type is the dominant microbialite
there is no interference with the fibrous crystallite fabric. Aggregates of
portion of the reefs and was studied in greater detail. The youngest car-
brownish,coccoidremainsthatmightrepresentformercoccoidcyanobac-
bonate veneers are formed by red-algal crusts with a thin living layer of
teria have been observed in only a few cases and are restricted to the
red algae on top, covered by living biofilms, green algae, and sponges.
contactbetweenthefibrousaragonitelayersandthesucceedingamorphous
Detaileddescriptionsofthethreefaciesfollow.
Mg–Si layer (Fig. 2D). They occur in depressions of the fibrous layer
‘‘Serpulite.’’—This facies type was observed directly covering basalt
below,buttherelativetimesequenceofthedifferentlayersandthecoccoid
boulders and is also known from pit sections between the reef heads
microfossilsremainsunclear.Inanycase,thethin,amorphousMg–Silay-
(Kempe and Kazmierczak 1993; Kempe et al. 1996). The highly porous
ers show a sharp contact with the fibrous aragonite layers below. This
framework consists of serpulid tubes (250 (cid:109)m–1.8 mm inner diameter),
contactcommonlyshowsscallopedmorphologies,whichareconsideredto
which are encrusted by smaller coiled Spirorbis tubes. Open voids are
representdissolutionpits(Fig.2B).Additionalorganicinclusionsthatoccur
partly filled with volcanicdetritus(feldspar,augite),foraminifera(mainly
within the aragonite include a few, single, boat-shaped diatoms less than
Miliolidae) and small gastropods. Fibrous aragonite cement of varying 20(cid:109)mlong(Fig.2B).Theremainingvoidsbetweenthegreen-algal-stro-
thickness (10 to 250 (cid:109)m) locally occurs inside the tubes, predominantly
matolitic framework are partially to completely filled with micropeloidal
inthesmallerones.Aspatialinterfingeringwiththeoverlying‘‘green-algal
sediment with abundant pellets, skeletal detritus, and siliciclasticdetritus,
microbialite,’’ as indicated by fibrous microstromatolitic crusts upon ser-
cementedbyanamorphousmatrixorfibrousaragonite(Figs.2A,3A,4).
pulitetubesandbymicropeloidalvoidsediment,wasobservedinonethin
Electronmicroprobeanalyses(twolinesections:53pointmeasurements)
section. The depth range of the serpulite facies is unknown. Marine bi-
reveal that micropeloids and fibrous cements of the voids consist of ara-
valves (Pteroidea) with Spirorbis tubes from soft sediments between the
gonite. Cryptocrystalline to amorphous matrix parts are composed of an
reefs at 15 m depth probably correspond to the serpulite. The serpulite
undeterminedMg–Siphaseidenticaltotheonementionedabove.Incon-
facieshasbeenconsideredbyKempeetal.(1996)asamarineinterstage, trast, pellets show a high-Mg calcite composition with (cid:59) 23 mole %
possiblycausedbythepercolationofseawaterthroughthecraterwalldur- MgCO .
ingapasthighsea-levelstage. Man3ymicrobialitesamplestakenfromthesurfaceoftheseasonallyex-
‘‘Green-Algal Microbialite.’’—This facies type corresponds to the posedreeftopsshowpoorlydevelopedmicrostromatoliticcrustsveneering
‘‘stromatolitic-siphonocladalean’’andthe‘‘peloidal’’zoneofKempeand green algal filament casts (Fig. 3A). Instead, a micropeloidal framework
Kazmierczak (1993), because these two zones grade into each other ver- composed of irregular, fibrous aragonite aggregates is developed. Arago-
ticallyandlaterally.The‘‘green-algalmicrobialite’’overliesthepreviously nite-cemented casts of siphonocladalean algae without stromatolitic en-
described,older‘‘serpulite’’withoutsharpboundaryandisatleast20cm crustationoccurinreef-topsamples,whereasgreen-algalmoldsareabsent
thick.KempeandKazmierczak(1993)reportamaximumthicknessof60– atgreaterdepths(15m).Somereef-topsamplesshowaragonite-cemented
80 cm.‘‘Stromatolitic-siphonocladalean’’samples(Fig.2A–E)havebeen accumulations of diatoms between the green-algalmolds(Fig.5A).Elec-
collected only from the reef tops, whereas samples of ‘‘micropeloidal’’ tronmicroprobeanalysesindicatethatthecommonlybrownish-coloredcell
limestone(Fig.3A–C)arepresentfromthereeftopdownto15mdepth. wallsofthegreenalgaearepermineralizedbyaMg–Siphase,thoughthe
The basic framework of this facies is formed by tufts and bushes of microprobesamplingareacoveredthe2-(cid:109)m-thickcellwallsandadjacent
erect, locally entangled tubes of siphonocladalean green algae (Figs. 2A, aragonite (Fig. 5B, C). On the basis of this observation, a Mg–Si permi-
3A).Thesearepreservedasmolds100to200(cid:109)mindiameter,eitheropen neralizationofcellwallsispossiblyanexplanationforthepreservationof
orpartlytocompletelyfilledbyisopachoustobotryoidalfibrousaragonite. coccoid cell remains (Fig. 2B, D–E) in subfossil microbialites of Lake
Thecell wallsand boundariesareevident at thebasalcontactsofthece- Satonda.
ments. In addition, some tufts show constrictions at cell boundaries. Di- Aggregatesofaragonitemicropeloidsarecommonly25–100(cid:109)minsize
chotomousbranchingisobservedrarelytoabundantlyinthedifferenttufts. and show dark microcrystalline centers with radiating, light, aragonitefi-
The outer surface of the green algal filament molds is given by fibrous bers(Fig.3A).Locally,apartialsilicificationofthemicropeloidalcarbon-
microstromatolitic crusts, cemented void sediment, or (in a few samples) ate(Fig.3B,C;belowred-algalcrust)preservedcoloniesofpleurocapsa-
bymicrocrystallinetocryptocrystallineveneersthatareupto10(cid:109)mthick. leancyanobacteriabetweenthearagonitemicroclots.Itisnoteworthythat
The characteristic microstromatolitic crusts that veneer the green algal thearagoniteisnotpresentasapermineralizationofcyanobacterialcolony
moldsare0.5to1.3mmthickandarecomposedofupto20fibrouslayers, sheathsbutoccursintheformofseparatemicroclots(Fig.3C)asobserved
each20to60(cid:109)mthick(Fig.2A–D).Eachlayerstartswithacryptocrys- inrecentreef-topbiofilms.
tallinetoamorphousbase(oftenlessthan5(cid:109)mthick)uponwhichfibrous At present-day reef surfaces, the whole fabric is affected by younger
aragonite nucleated (Fig. 2B–D). The fibrous parts are finally terminated dissolutionprocesses,whichhaveresultedinenlargementofprimaryvoids
by a smooth, undulating surface. The next layer startsagain with acryp- and truncation of fabrics. The younger fibrous aragonite cements discon-
tocrystalline to amorphous base. One to three of the cryptocrystalline to tinuously line the voids and smooth the microrelief. With regard to their
amorphouslayersreachupto60(cid:109)mthicknessandcanbetracedthoughout fabric and chemical composition, these younger cements are identical to
thecrustsofathinsection. themicrostromatolites.
MICROBIALITEFORMATIONINSATONDACRATERLAKE 109
FIG.2.—Subfossilreefcarbonates.A)Mainpartofthereefcomposedofmicrostromatoliticcrusts(strom)encasingformerfilamentsofsiphonocladaleangreenalgae
(green)similartotherecentCladophoropsis.Frameworkvoid(void)ispartlyfilledbymicropeloidalsediment,fecalpellets,andskeletalandsiliciclaticdetrituswithinan
amorphousormicrocrystallinematrix.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).B)High-magnificationviewofmicrostromatolitelaminaealternation.
Notethesharpbasalcontact(dashedline)oftheMg–Silayer.Theassociatedpits(pit)arecuttingintothefibrousaragonite,probablyindicatingdissolutionpriortoor
concurrentwiththeformationoftheMg–Si-phase.Noteelongateddiatomremains(dia)withinthetopofthefibrousaragonitelayer.Dryshore,reef#1.Transmittedlight.
SampleSat93/74(1813).C)Microstromatoliticcrust(strom)composedoffibrousaragonitelayers(light)andthinlaminaeofanamorphoustomicrocrystallineMg–Si
phase(dark).Totheleft,thebasalcontacttoaformergreen-algalfilament(green)isvisible.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).D)Detailof
partCshowingfilamentousstructures(fil)ofsupposedfungalorigincross-cuttingthefibrousaragonitefabricofthemicrostromatolites.Thefilamentousstructuresare
interpretedtobeofendolithicorigin,thereforeshouldbedestructiveratherthaninvolvedinconstructiveprocessesofcrustformation.Noteremainsofcoccoidmicroor-
ganisms (cocc) within the top of one of the aragonite layers. Such coccoid remainsare rare and mightresult form coccoid cyanobacteria,althoughtheirroleincrust
formationremainsinterpretive.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).E)Remainsofcoccoidmicroorganisms(cocc)withinthebasalpartofa
fibrousaragonitelayer.Thesespheresmayresultfromcoccoidgreenalgae,cyanobacteria,orspores.Becauseofthescatteredarrangementoftheremains,anoriginfrom
benthiccoccoidcyanobacteriaisconsideredunlikely.Thelarge‘‘sphere’’isanartificialbubbleinthesection.Dryshore,reef#1.Transmittedlight.SampleSat93/74
(1813).
110 G.ARPETAL.
FIG.3.—Subfossilreefcarbonates.A)Green-algal-microstromatoliteframestoneshowingerectgreen-algalfilamenttubes(green)encrustedbymicrostromatolites(strom),
and voids filled with aragonitic micropeloids (pel) within a Mg–Si matrix. Note fan-shaped aragonite cements (cem) that formed within voids. 0.3 m depth, reef #1.
Transmittedlight.SampleSat96/14.B)Micropeloidalaragonitelayer(microclots),containingsilicifiedpleurocapsaleancyanobacteria,overlainbyared-algalcrust(pey).
7mdepth,reef#1.Transmittedlight.SampleSat93/6.C)Silicifiedpleurocapsaleancolonies(pleu)betweenaragonitemicroclots(arag)ofthemicropeloidallayershown
inpartB.7mdepth,reef#1.Transmittedlight.SampleSat93/6.
Conspicuousstructuresassociatedwithmicrostromatoliticcrustsandmi- crusts are missing. Electron microprobe analyses (Appendix 3, see Ac-
cropeloidal parts in semicryptic voids of Satonda reefs are semiglobular knowledgments) indicate an aragonite mineralogy for Peyssonnelia thalli
structures (Fig. 6A, B), which have been compared to the Paleozoic mi- andfibrouscements,high-Mgcalciteforfecalpellets,micritefillingswithin
croproblematicum Wetheredella (Kazmierczak and Kempe 1992). The Peyssonneliathalli,andnubecullinidforaminifera,andamorphousMg–Si
semicircular to halfmoon-shaped sections are 90–190 (cid:109)m in height and formatrixparts.
120–290(cid:109)minwidth.Oneortwoaragonitearrays,whichoriginateatthe Afinal,dense,smoothtodendroidcrustcomposedofthesquamariacean
basalsubstrate,formthesestructures.Theouterlimitissharplydefinedby Peyssonnelia, the coralline red alga Lithoporella, and nubecullariid fora-
adarklineoralessthan5(cid:109)mthickorganicwall(Fig.6B). miniferaformovergrowthsonthecornflake-likered-algalcrustsandolder
‘‘Red-Algal–Foraminiferal Crusts.’’—Red-algal–foraminiferal crusts microbialites (Fig. 3B). The Peyssonnelia thalli commonly show a light–
formtheyoungestpartsoftheSatondareefs.Theyveneerolderreefparts darklaminationduetoalternatingcellsizes.Layersofsmallcellsthereby
fromtheseasonallowstandleveldowntobelowthechemoclineat22–24 appear ‘‘micritic’’ at first glance but are composed of aragonitic microfi-
mdepthandcorrespondtothe‘‘cyanobacterial–red-algalzone’’ofKempe bers. Mineralized coccoid cyanobacteria have been observed in the hori-
and Kazmierczak (1993). The highly porous, cornflake-like framework is zontal crevices between the thalli. However, all of these coccoid cyano-
composedoffoliaceousthalliofthesquamariaceanredalgaPeyssonnelia bacteria were preserved by silification (amorphous Mg–Si phase) and no
withirregulartolenticular,millimeter-sizevoidsinbetween(Fig.6C).The CaCO permineralization was found. Electron microprobe analyses per-
3
voidsarepartiallyorcompletelyfilledbyaragonite-cementedfecalpellets, formedonthisfinaldensered-algalcrustconfirmthepreviouslydescribed
miliolidforaminifera,raregastropods,andpatchesofmicropeloids.Lower compositionofskeletons(Fig.7).AragoniticPeyssonneliathallialternate
sides of Peyssonnelia thalli are characterized by hypobasal aragonite bo- with high-Mg calcitic Lithoporella thalli and nubecullinid foraminiferal
tryoids that are marginally micritized (Fig. 6C; Kempe and Kazmierczak tests.TheamorphousMg–Siphaseisrestrictedtohorizontalcrevicesnear
1993).Thesehighlyporouscrustsarecommonly5cm,andlocally15–25 thereefsurfaceandafewsmallporespaces,butitalsooccursasapartial
cm,thickatdepthsbelow5m(KempeandKazmierczak1993).Thethick- silificationofMg-calciticLithoporellathalli(Fig.7).
ness of these crusts decreases downwards to 3–4 cm close to the chem- The uppermost living thalli of the final dense red-algal crust represent
ocline. In shallow reef parts (0.3 m below seasonal lowstand) red-algal the presently growing part of the reefs. In shallow water this final crust
MICROBIALITEFORMATIONINSATONDACRATERLAKE 111
FIG.4.—Electronmicroprobetraverseofamicrostromatoliticcrustofthesubfossilreefcore.SampleSat93/74(1813).Dark-appearinglayersarecomposedofaragonite,
whereasthin,lightlayersareformedbyanunidentifiedMg–Siphase.High-Mgcalciteisrestrictedtointernalsedimentofpocketsbetweenthemicrostromatolites.
112 G.ARPETAL.
FIG.5.—Electronmicroprobesectionofasubfossilgreen-algal-microbialitesampledataseasonallyexposedreeftop.SampleSat96/18,0.5mdepth,reef#1.A)Overview
of the green-algal-microbialite section. Spaces between green-algal moulds are infilled by halfmoon-shaped diatoms encased in aragonitecement. Whitelineindicates
transectinpartB.B,C)Transectacrossmarginalpartsofgreen-algalmolds,theirwalls,andthecarbonatebetweentwofilaments.Measurementpoints23and32are
interpretedasmixedsignalsofanamorphousMg–Siphaseandthesurroundingaragonite(noteSrcontents).
locally veneers directly the ‘‘green-algal microbialites,’’ separated by a alean green algae. The squamariaceanPeyssonneliadominatesinshallow
corrosion plane. Crusts dominated by nubecullariid foraminifera protrude water,whereasthecorallinaceanLithoporellaisincreasinglyabundantwith
even into near-surface cavities of the subfossil cornflake-like red-algal depth.However,livingthalliofbothtaxaarepresentthoughoutthiszone
frameworkandcorrodedgreen-algalmicrobialites. (Fig.8A,B).Insectlarvaltubes,whichoccurregularlyatshallowdepths,
areveneeredbythered-algalthalli,also.Reef-surfacebiofilmsofthered-
Reef-Surface-LivingBiotaandBiofilmsoftheDrySeason algalcrustsarediscontinuouslydeveloped,usuallylessthan10(cid:109)mthick,
andcompriserod-shapedandfilamentous,non-phototrophicbacteria(Fig.
Samplesfromthelivingreefsurfacewereobtainedfromthewaterline
8E,F).Heterotrophicbacteria,whichdigestcellwallsofdeadgreenalgal
downtobelowthechemoclineat22mdepth(Appendix2).Owingtogaps
filaments, have been detected by TEM sections (Arp et al. 1996). In ad-
in the sampling profile, boundaries or transitions cannot be assigned to
dition,fungalhyphae,whichpenetratefilamentsofcyanobacteria,arealso
defineddepths.
present(Arpetal.1996).Hadromeridsponges(Laxosuberitessp.)locally
Cyanobacterial and bacterial biofilms on living red algae are generally
less than 10 (cid:109)m thin and grow preferentially within depressions or sub- veneerthelivingordeadred-algalthalli.Theircontactwiththecalcareous
red-algalsubstrateisalwaysmediatedbyathinbiofilmofnon-phototrophic
surfacevoidsofthered-algalcrusts.Itisimportanttonotethatallinves-
tigated biofilms generally comprise a large portion of non-phototrophic bacteria.
microorganisms,aboveallfilamentousbacteria.Inaddition,fungalhyphae Phototrophic microorganisms, such as pennate diatoms, coccoid algae
and numerous coccoid and rod-shaped bacteria are present, especially at (Fig. 8E, F; ‘‘Dermocarpella’’), and the cyanobacteria Pleurocapsa and
decayingspongetissues.Threezonesbetweenthelake-levellowstandand Phormidium spp., occur only scattered on the red-algal surfaces. Only in
thechemoclineandonebelowthechemoclinearedefined. depressions of the surface and at the surface of green algal filaments are
Peyssonnelia–Lithoporella Zone.—Reef surfaces betweentheseasonal cyanobacteriaanddiatomsmorecommon,alwaysassociatedwithnon-pho-
lowstand and approximately 7 m depth are characterized by living Peys- totrophic bacteria. In contrast, several samples showabundant pleurocap-
sonnelia–Lithoporellacrustsandadensemeadowofattachedsiphonoclad- salean cyanobacteria in subsurface voids between the living crustosered-
MICROBIALITEFORMATIONINSATONDACRATERLAKE 113
FIG.6.—SubfossilWetheredella-likestructuresandreefcarbonates.A)Subfossilcystousstructures(weth)formingacrustwithinacavityofthered-algalreefframework
(red).Thesecystousstructures,superficiallyreminiscentofthePaleozoicmicroproblematicumWetheredella,areconsideredhereintobefossilizedrestingbodiesofsponges.
Theremainingporespaceisfilledbyaragonite-cementedmicropeloids(pel).Dryshore,reef#1Transmittedlight.SampleSat93/44.B)Close-upviewofWetheredella-
like structures showing radial-fibrous aragonite and a distinct, defined wall (wall). Dry shore, reef#1.Transmitted light. Sample Sat 93/44. C)Foliaceousthalliofthe
calcifiedsquamariaceanredalgaPeyssonnelia(pey)forma‘‘cornflake-like’’reefframeworkintheyoungerreef.Lowersidesofthethallioccasionallyshowhypobasal
aragonitebotryoids(botr).Theremainingirregulartolenticularvoidsarepartlyfilledbyaragonite-cementedmicropeloids(pel),fecalpellets(faec),andfibrousaragonite
cements(cem).24–25mdepth,reef#10/11.Plane-polarizedlight.SampleSat93/28.
algal thalli (Fig. 8B). In addition, endolithic cyanobacteria of the Hyella diameter)occurinvaryingabundancealso.LivingLithoporellamonolayers
group, which bore in living and dead Peyssonnelia thalli, have been ob- are still present, but less abundant and discontinuously distributed within
servedinonesamplefrom0.5mdepth(Fig.8C,D).Pleurocapsacolonies the biofilm. Siphonocladalean green algae have not been found at that
areabundantonlyincrevicesbetweenthered-algalthalli,buttheyremain depth,buthadromeridspongesoccuratleastatupto18mdepth.
softand unmineralized (Fig. 8B).Green-algalholdfastsarelocatedinthe Non-PhototrophicBiofilmZone.—Onthebasisofonesampletakenat
depressions or are already overgrown by the calcareous red-algal thalli. 24–25mdepth,reefsurfacesbelowthechemoclinearecomposedofdead
Suchenclosedgreen-algalfilamentsformtheonlyplaceswherenon-skel- Peyssonnelia–Lithoporella crusts that are veneered by 10–20 (cid:109)m thick,
etal aragonite precipitation was observed in one single sample (Fig. 8A). detritus-rich, soft biofilms of non-phototrophicbacteria.Thedetrituscon-
Fibrous aragonite formed inside of lysed green-algal cells, whereas ara- sists of organic particles, siliciclasticgrains ((cid:44) 10 (cid:109)m) and few Mnhy-
gonite precipitates were observed neither in adjacent biofilms of the reef droxide particles. Microorganisms are dominated by filaments more than
surfacenorincrypticbiofilmsintheirvicinity.Crevicesandvoidsofthe 60(cid:109)mlongand0.4–0.5(cid:109)mthick,ofsupposedbacterialorigin.Nobranch-
red-algalframeworkarecommonlyrichinflocculentorganicmaterial(de- ing has been observed in these filaments. In addition, numerous coccoid
caying sponge tissues, organic detritus) and cyanobacteria, and reveal an (0.65 (cid:109)m) and rod-shaped (0.25–0.5 (cid:109)m diameter; 1 (cid:109)m long) bacteria
extensivepopulationofcoccoidbacteria,whichapparentlydecomposethe are present in the biofilm. Rare spirillae (0.5 (cid:109)m; 3.25 (cid:109)m long) and
organics. No carbonateprecipitateswereobservedinassociationwiththe coccoid phototrophs (?cyanobacteria, 2.3 (cid:109)m in diameter) occurin small
bacterialdecompositionoforganicmatter.Figure9isaschematicdrawing groupsofeightcellsandless.
that summarizes the observations on samples from the reef-surfacecrusts
ofthePeyssonnelia–Lithoporellazone.
Reef-Surface-LivingBiotaandBiofilmsoftheWetSeason
Pleurocapsa–‘‘Dermocarpella’’ Zone.—Deeper-water samples (14–18
m depth) have surfaces that are composed largely of dead Peyssonnelia Thezonationofthereef-surfacecommunitiesinJune1996differedfrom
thalli that are overgrown by almost continuousPleurocapsa–‘‘Dermocar- thatof October1993.ThelakelevelinJune1996, afewweeksafterthe
pella’’biofilms.ErectandprostratePhormidiumspp.(0.75(cid:109)mand2(cid:109)m end of the rainy season, was still approximately 0.4–0.5 m higher than
114 G.ARPETAL.
FIG.7.—Electronmicroprobetraverseofared-algal-foraminiferalcrustofthereefsurfaceat17mdepth(reef#1).AragoniticPeyssonneliathallialternatewithMg-
calciticLithoporellathalli.AnamorphousMg–SiphaseoccursinvoidsandcrevicesbutalsopartiallyreplacesMg-calciticLithoporellaskeletons.